1. Field of the Invention
The present invention relates to minimizing errors in the components produced by an extreme ultraviolet lithography (EUVL) system, and more specifically, it relates to a method and apparatus for repairing surface height errors on precision reflective optics.
2. Description of Related Art
The primary motivation of this invention is to enable the efficient correction of surface height errors on precision reflective optics. One of the limiting technologies is the fabrication of optical surfaces (mirrors) that are geometrically accurate to very stringent tolerances, about 0.1-0.2 nm rms. An important means of describing and/or categorizing these errors is the by their spatial scale, i.e. “how wide are they”. The spatial scale of the figure errors relevant to this invention ranges from long-range errors such as astigmatism to millimeter-sized ripple. It is important to be able to address all of the spatial scales of the errors.
For lithographic optics, it is important to control the errors for all size ranges. One way to characterize the spatial sizes of the errors is to look at the Fourier transform of the errors. Beginning with a 3-D map of the surface indicating the height errors, the 2-D Fourier transform is calculated, and then the Power Spectral Density (PSD) is calculated. The PSD provides a measure of the error (rms power) in terms of spatial frequency or spatial period. For stringent lithographic applications, a need exists to control the height of the PSD for a very wide range of spatial periods, e.g., from 100 millimeters to 20 nanometers. This invention addresses the range from about 1 m to about 0.5 mm, which is often loosely termed the figure of the optic.
The problems with current fabrication processes are mainly that they cannot address this entire range of spatial periods, nor address sub-ranges without causing errors within other ranges. The current invention offers a unique opportunity to remove errors within the entire range termed figure or within sub-ranges of sizes without causing errors in other size ranges.
Current methods also cannot offer an effective way of removing waviness errors in the 1 mm range. The current invention offers a straightforward method of addressing the errors with a very small tool footprint that can naturally address such small range waviness.
The principal method currently used in the optical industry for fabricating precision optical surfaces is small tool polishing. This generally involves an abrasive slurry with some type of rotating or oscillating tool that causes the slurry to abrade the optical surface. In some cases, the slurry is chosen so that the main method of removing material is a chemical reaction. The tool acts to localize the activity of the chemical reaction by adding localized pressure or relative velocity. In other cases, the action of the slurry can be mechanical in that the slurry particles physically scrape away material. Often, the slurry polishing process is referred to as chemo-mechanical, indicating that it is a combination of chemical and mechanical material removal mechanisms. Small tool polishing is in current application in the lithographic optics industry.
The location of material removal for small tool polishing is determined by the localized pressure distribution provided by the polishing tool. This tool might comprise a pad on the end of a rotating spindle that contacts the workpiece with a controlled loading force. The specific geometry of the polishing pad and its orientation with respect to the surface typically comprise proprietary information. The specific choices of pad and slurry are also typically kept as proprietary information.
Other material removal mechanisms are also in use or have been considered. Ion beam figuring (IBF) involves sputtering away the surface atoms of an optical surface by ion impingement with sufficient energy to break chemical bonds. A geometrically well-controlled broad ion beam is created with an ion gun, often with a near gaussian profile. This beam is then scanned over the optic, leaving a path of material removal.
Another technique is plasma-assisted chemical etching, where a low energy plasma is generated near the optical surface. Reactant gases are fed into the plasma zone where chemically-reactive species are formed. These species react with the optical surface and form products that are then removed by the vacuum system. It is essential that the vapor pressure of the products is sufficiently high so that they will flow into the vacuum system for removal. In general, plasma-assisted chemical etching involves sufficiently low ion energies such that sputtering does not occur. Ion beam figuring requires ultra-high vacuum, while plasma etching requires only a mild vacuum.
Controlled deposition of surface layers is another technique that has been considered for correcting surface height errors. In this technique, a material such as silicon dioxide, is deposited on the surface to add height in areas that are deemed too low. The location of the deposition can be controlled by either placing a mask over the surface that only allows deposition through “open” areas, or by scanning a beam of deposition over the surface. In the latter case, the beam of deposition is analogous to the beam of removal mentioned above for ion beam figuring. The general type of equipment used for controlled deposition process is similar to equipment used for depositing coatings on optical elements.
Small tool polishing, ion figuring, plasma-assisted chemical etching, and controlled deposition are considered convolution processes. The location where removal (or addition) of the surface takes place is dictated by the location of the tool. The removal characteristics of the tool are described by a footprint function, which might be measured by simply letting the tool dwell at a location on the surface for a given period of time. A measurement of the resulting dimple in the material might reveal that the shape of the removal zone is gaussian, although other shapes may also be created. In general, the tool is planned to move over the optical surface and remove material, dwelling longer where more removal is required, less where less removal is required. The convolution concept is summarized such that the amount of removal at any one point on the surface is due to the sum of the removal contributions from all of the positions of the tool where the footprint overlapped with the point. There are relatively sophisticated mathematical algorithms to determine what path the tool should have over the part, and how long it should dwell at each location along its path.
There are several problems that have limited the above removal techniques from correcting errors in precision optics. First, it is difficult to address small-scale waviness because the footprint of the tool is generally larger than the 1 mm sized waviness that is part of the range of sizes that are of interest. Also, the control and stability of the shape of the footprint is not repeatable to the degree desired for precision removal.
Also, when addressing a broad range of spatial scales of errors, e.g., ranging from 1 mm to 100 mm waviness, different sized tools are used; larger tools are used for larger errors. It is observed in practice that when using a given size tool to address a given size error, that it often increases errors in other size ranges. Thus, the removal process often results in chasing the errors from one size range to another.
A particularly troublesome example of creating errors in another size range is in the fabrication of optics for EUV or x-ray optics. The surfaces of these optics require atomic level smoothness for spatial scales smaller than a micron (termed “microroughness”). In general, the final application of the above-mentioned removal processes shows a tendency to increase the roughness in the domain of these small scale errors. Thus, attaining acceptable levels of figure errors are typically attained at the expense of low levels of microroughness.